The present invention relates to a method and apparatus for removing thimbles from the stubs of anodes, such as anodes used in the electrolytic smelting of metals such as aluminum.
A carbon anode block, used in the electrolytic smelting of metals such as aluminum, is supported by yoke stubs attached to an anode rod. As shown in FIG. 1, each yoke stub 10 has a thimble 12 fitted at its end, and the thimbles 12 may be cast within recesses in the carbon anode block. During the electrolytic smelting of metals, the carbon anode block is consumed. To replace the consumed carbon anode block, the anode remnants and the thimbles are removed from the anode yoke stubs with a machine known as a thimble press.
A conventional thimble press generally includes a single ram that is used to push the stubs through the thimbles, thereby removing the thimbles from the stubs. In operation, as shown in FIG. 2, a clamp 14 is placed around each stub 10, with the opening of the clamp 14 small enough to prevent the thimble 12 attached to the end of the stub 10 from passing through. The ram 16 then pushes the stubs 10 (and the thimbles 12) towards the clamps 14. When a thimble 12 comes in contact with a stationary clamp 14, the movement of the thimble 12 is stopped, and the movement of the stub 10 continues, allowing the clamp 14 to stripe the thimble 12 from the end of the stub 10.
One of the disadvantages of conventional thimble presses is that the removal of the thimbles using a single ram may generate a significant bending moment in the yoke, which may either damage the yoke or shorten its operating life. This occurs when the ends of the stubs are not even (not unusual under normal working conditions) and thus don""t come in contact with the cam at the same time. For example, in the case illustrated in FIG. 2 where the middle stub 10 is longer than the two side stubs 10, the ram 16 can only push against the middle stub 10, and the removal of the two side thimbles 12 generates a bending moment in each of the yoke arms. For example, if 5,000 kg of force is required to remove a thimble and the distance between the middle stub and each of the side stubs is 1 meter, the removal of the side thimbles generates a bending moment of 5,000 kg-m in each of the yoke arms.
The method and apparatus embodying the various aspects of the present invention overcome the problems associated with conventional thimble presses.
In accordance with one aspect of the invention, an apparatus for removing thimbles from stubs of an anode yoke includes a number of separate rams and a ram control system. The rams preferably are capable of independent movements, which are controlled by the ram control system. The ram control system has two modes. In the first mode, each ram is advanced towards a stub, and the ram force is limited to a preset maximum value. When the ram has contacted the stub, the advancement of the ram may be stopped because the maximum ram force may be insufficient to move the stub. In the second mode, the rams are advanced at about the same speed, preferably at the same speed, to push the stubs through the thimbles, thereby removing the thimbles from the stubs. Preferably, the ram control system remains in the first mode until each ram has come in contact with a ram and/or until the force applied by each ram against the stub has reached the present maximum value.
In accordance with another aspect of the invention, a method for removing thimbles from stubs of an anode yoke includes the step of advancing a ram towards each stub with the force applied by the ram against the stub being limited to a preset maximum, and another step of advancing the rams at the same speed, preferably at the same speed, to push the stubs through the thimbles, preferably while not limiting the force applied by each ram to the preset maximum force. In a preferred embodiment, the first step precedes the second step.
The present invention has a number of advantages over conventional thimble presses. For example, during the removal of the thimbles, a preferred embodiment of the present invention does not generate a sufficiently large bending moment in the yoke arms so as to damage the yoke or shorten its operating life. The reason for this advantage is that in a preferred embodiment of the present invention, a thimble is removed by pushing primarily against the stub on which the thimble is installed. In other words, the force used to remove a thimble is generated by the ram which is pushing the stub on which the thimble is installed. In conventional thimble presses, a thimble may be removed by pushing against one or more other stubs when the stubs are not even.